Continuous breath-by-breath analysis of a patient's respiratory gases in the operating room is becoming increasingly important in improving patient safety during anesthesia. Respiratory and anesthetic gas monitoring, as well as the determination of specific cardiac and pulmonary functions which are based upon the uptake and production of specific tracer and respiratory gases, has reached a high standard of technological advancement with the development of sophisticated sensors, transducers and computers. These monitoring techniques enable quick diagnosis and treatment of unfavorable trends in the condition of a patient and lead to an improved survival rate, early extubation following surgery and a shorter time in the intensive care unit. Applications of respiratory gas and anesthetic agent monitoring include the measurement of oxygen consumption, carbon dioxide production, anesthetic agent uptake and the possibility of detecting anesthesia machine circuit disconnections and introduction of air emboli into the blood. These measurements lead to a more scientific basis for the administration of anesthetic agent. breath-by-breath monitoring of a patient's respiratory gases and simultaneous determination of multiple specific respiratory gases and anesthetic agents in the patient's system in the intensive care unit and other critical situations, can often facilitate diagnosis and treatment, anticipate and prevent the development of oncoming problems, and otherwise provide instant data for physicians and other health care personnel to use in therapeutic situations. The same may be said of the breath-by-breath analysis of gas mixtures used for non-invasive determination of cardiac output and lung function.
Respiration monitoring of the critically ill patient is now available in operating rooms and intensive care units utilizing the techniques of mass spectrometry to identify specific volatile anesthetic agents and quantify nitrogen. Multiple-bed sampeling techniques make feasible the use of an expensive, multiplexed mass spectrometer gas analyzer because it can be shared among a number of patients. Since the mass spectrometer unit is large and not easily moved from room to room, it is generally placed in a remote location and lengthy capillary tubes are used to connect the patients to the unit. This tube transport system increases the possibility of gas sample mixing, time delay, waveform distortion and disconnections, and poses inherent limitations for use in anesthesia, critical care and medical research. Furthermore, the mass spectrometer requires a vacuum system, which increases its cost and maintenance and decreases its reliability. The vacuum system also prevents use of the mass spectrometer in many situations where Helium or other inert gases are present. Inert gases are often used in laser surgery procedures to purge the location where the laser is applied to the patient. Diffusion of the inert gas into the mass spectrometer's vacuum system renders it virtually useless for respiratory gas analysis.
Alternatively, there are a variety of gas detectors based upon several different physical principles including infrared absorption, and polarographic and solid-state semiconductor analysis which, when taken together, can measure anesthetic agents and respiratory gases. Some disadvantages of these detectors are high aggregate cost, bulkiness, slow response time and poor data integration into one comprehensive display of patient parameters.
An alternative technique proposed for use in simultaneously monitoring several gases in critical care situations is based on Raman light scattering. The Raman light scattering effect occurs when monochromatic light interacts with the vibrational/rotational modes of gas molecules to produce scattered light which is frequency shifted from that of the incident radiation by the amount corresponding to the vibrational/rotational energies of the scattering gas molecules. If the incident light photon loses energy in the collision, it is re-emitted as scattered light with lower energy and consequently lower frequency than the incident photon. This inelastic scattering is termed Stokes Raman scattering. Similarly, if the incident photon gains energy in the collision, it is re-emitted as scattered light with higher energy and consequently higher frequency than the incident photon. This type of inelastic scattering is termed anti-Stokes Raman scattering. Since these energy shifts are species-specific, an analysis of the various frequency components present in the Raman scattering spectrum provides chemical identification of the gases present in the scattering volume. The intensity of the various frequency components or Raman lines provides quantification of the gases present, providing suitable calibrations have been made. The relative sensitivity to the different gases remains absolutely fixed, eliminating frequent calibration requirements. In principal, either Stokes or anti-Stokes Raman light scattering can be utilized. However, at room temperature, the Stokes Raman effect is generally more intense.
Raman techniques have been widely used for atmospheric monitoring and for combustion applications. Sensitivities better than 1 ppm have been demonstrated. A typical application of Raman scattering analysis coupled with computer-assisted signal processing techniques is reported in Lapp, et al., "Laser Raman Gas Diagnostics," Plenum Press, New York/London, 1974.
Raman scattering analytical techniques are also described in the patent literature. Benner, et al. (U.S. Pat. No. 4,648,714) contains a summary of many of these, including Chupp (U.S. Pat. No. 3,704,951), Hatzenbuhler (U.S. Pat. No. 3,807,862) and Leonard (U.S. Pat. No. 3,723,007).
A system for the simultaneous detection of multiple gases is taught in Albrecht, et al., German Pat. No. DE 27 23 939 C2 (1983). This patent discloses an extracavity multi-pass sample cell having two concave mirrors which constrain an unpolarized laser beam in a region between the two concave mirrors. The two concave mirrors are oriented so that the laser beam repeatedly reflects back and forth between the two mirrors through a focused region, or region of increased light intensity. It is from this focused region that Raman scattered light from the sample under analysis is collected and analyzed. The scattered light from the focused region is collected by a series of six detector channels mounted about the focus region to provide a 360.degree. equatorial monitoring geometry for the Raman scattered light. Each channel has collection lenses, interference filters and photon detectors. The interference filters are selected so that each channel is sensitive to a specific gas. This is accomplished by combining a broadband filter with one gas-specific filter. The six channels thus collect signals from six separate Raman lines for the simultaneous monitoring of six different gas components. This method, wherein the detector channels are arranged in an equatorial plane and are aimed at a single point of high light intensity within the cell is, by its own admission, limited to no more than six detectors due to the fact that the effective solid angle per channel for collection of Raman light from the sample corresponds to approximately an f/1 collection lens. Utilization of such a collection lens is alleged to ensure optimum exploitation of light scattering. If it is desired to monitor more than six gas components, the registered light scatter is split into several gas-specific components which are successively routed to various detectors. This is accomplished using a collection lens which directs the scattered light onto a series of interference filters and concave mirrors at an angle of incidence such that the gas to be detected at the first filter passes through to a detector with the remaining light being reflected to a concave mirror and passed to another filter through which another specific light component is filtered out. The process continues for as many filters, detectors and mirrors as required to obtain the desired number of channels. A mirror which directly reflects light from the last filter in the serial chain passes the remaining light back through the chain so that the entering and exiting directions coincide, and the light passes all filters again precise positioning of the angles of the filters and mirrors. Moreover, there is a cumulative loss of light intensity with each reflection which, after reflection from several filters and mirrors, becomes quite significant. Since Raman scattered light is considerably weaker than the incident light, it is desirable to direct, collect, filter and focus each monitored wavelength of the scattered light to the detector with a minimum number of refractions and reflections. The equatorial sampling plane geometry also requires that the laser beam be unpolarized in order to obtain adequate and uniform Raman signals from the gas sample.
An improved system for the near-simultaneous determination of multiple polyatomic gases by collection and detection of Raman scattered light which overcomes many of the limitations of the Albrecht system is disclosed by Benner, et al. (U.S. Pat. No. 4,648,714). This patent teaches a system and method, wherein a gas sample is placed in a sampling cell located in the resonant cavity of a laser. A polarized laser beam, having sufficient intensity to produce detectable signals of Raman scattered light, is passed through the cell and gas. Both inelastic Raman scattered light and elastic laser-scattered light are collected from a single focused region of the polarized laser beam by a collection lens having its optical axis perpendicular to both the axis of the laser beam and the polarization vector of the laser beam. Another portion of the scattered light is captured and redirected to the collection lens by means of a reflection mirror having an optical axis oriented perpendicular to the axis of the laser beam and located adjacent to the focused region. The mirror is external to the gas cell. The collected scattered signals are directed onto a multilayer dielectric laser line rejection filter where the scattered elastic laser signals are filtered out and the inelastic Raman scattered signals are transmitted to a rotating filter wheel containing a series of interference filters, each filter being specific to the transmission of one Raman line. The Raman lines passing through the rotating filters are sensed sequentially by means of a single detector, and amplified and converted into digital electrical pulses which are processed and converted into visual readouts which display the identity and concentration of each of the polyatomic molecules present in the gas being analyzed.
While the system disclosed in the Benner, et al. patent is a substantial improvement over prior art techniques, there are practical limitations associated with the use of a single detector and filter wheel containing numerous interference filters. For example, near-simultaneous serial sampling with a single detector requires a microprocessor fast and powerful enough to process all of the sequential data from "n" channels. It also requires that sufficient signal intensity be present at the detector for each filter position to enable the signal to be analyzed in a very short time period. Cross talk corrections between channels, e.g., the anesthetic gases and nitrous oxide channels, also require a fast microprocessor when the data are acquired serially.
A need thus exists for a device and method for simultaneously monitoring multiple gases using Raman scattering techniques which does not sacrifice performance when more than six detector channels are used. Such a device should also be capable of rapidly monitoring more than six gas species simultaneously without requiring ultra-fast electronic processors or exotic optical systems. The monitoring device should accomplish the foregoing without sacrificing the response time of the system or the accuracy of its determinations.